The invention relates to surface coating compositions having improved resistance to deterioration resulting from attack by microorganisms. More particularly, it relates to surface coating compositions that contain a biocidally effective amount of a substituted 1,3,4-thiadiazole.
It is well known that organic surface coatings such as paints, stains, and varnishes are susceptible to attack by microorganisms. Such attack causes discoloration, blemish and disfiguration on the surface of the dried coating and ultimate loss of adhesion of the coating to the substrate. This deteriorating effect is commonly known as mold or mildew growth.
Several fungi, yeasts, algae, and bacterial species are known to cause mold growth on exterior coatings. Most frequently found fungus colonies on painted surfaces are Aeruobasidium species. Secondary causes of discoloration are Alternaria, Cladosporium and Penicillium species in warm, humid regions and Aspergillus species in northern climates. To be useful as a mold inhibitor, the biocide must meet a number of criteria. The biocide must possess antimicrobial properties. The biocide must be compatible with the coating medium and retain its activity in the container during the normal storage life and in the dried coating during the service life of the coating.
The mere fact that a chemical compound is known to possess fungidical activity does not mean that it will be effective in inhibiting mold growth on exterior coating surfaces for long periods of time. In fact, most known fungicides are not useful for protecting exterior coatings. Many fungicides lose their fungicidal activity prior to being applied in a dried film form. Other fungicides may prevent deterioration by anaerobic microorganisms in a sealed can, but fail to prevent the formation of mold or mildew by aerobic microorganisms on a surface exposed to ambient air. The biological activity of many chemical compounds is impaired by the weathering environment to which most exterior coatings are exposed.
Other chemical and physical properties influence the activity of the fungicide such as solubility, dispersibility, film permeability, volatility and odor. In the dried coating, a mold inhibitor must retain its activity and remain effective under the influence of detrimental atmospheric conditions such as ultraviolet radiation and pollution, particularly sulfur type effluents. The inhibitor must remain in the coating without leaching out or evaporating to the atmosphere. For optimum effectiveness, the mold inhibitor must be uniformly distributed throughout the coating, that is, it must migrate throughout the film and may not concentrate in any one stratum of the coating.
Prior art references have suggested the use of several 1,3,4-thiadiazole compounds as paint fungicides and bactericides. Thus, U.S. Pat. No. 3,888,869 discloses thiocyanatoalkythio derivatives of 1,3,4-thiadiazole and U.S. Pat. No. 4,209,522 5-nitro-2-thiazolythio derivatives of 1,3,4-thiadiazole. However, none have proven entirely satisfactory for commercial applications.
Unexpectedly, it has been found that certain alkylthio derivatives of 1,3,4-thiadiazole satisfy the criteria for application in surface coatings and are highly effective in inhibiting mold growth on paint coatings.